The present invention relates to carbonaceous fibers having sharp ends, and to a process for producing such carbonaceous fibers.
More particularly, the invention relates to carbonaceous fibers which have sharp ends and which are useful as an electron source for field emission or the like, for example, in a cold-cathode type display device, and to a process for producing such carbonaceous fibers.
Recently, there has been investigated the use of carbonaceous fibers as an electron source of an electron-emitter in a cold cathode employed in a device such as an electronic display device or an imaging device.
With regard to processes for producing such carbonaceous fibers, Japanese Patent Application Laid-Open (Kokai) No.8-115652, for example, discloses carbonaceous fibers deposited through pyrolysis of hydrocarbon gas serving as a raw material in a microcavity formed between two electrodes, each electrode being disposed on an insulated substrate.
Japanese Patent Application Laid-Open (kokai) No. 10-112257 discloses a process for gas phase synthesis of diamond-like carbon, including steps of implanting carbon ions or carbon cluster ions on a substrate cathode surface to thereby form nucleating sites, and growing diamond-like carbon from the sites.
Although these processes can be carried out from a technical standpoint, the processes involving a thermal treatment step, however, have an adverse effect on the cathode material. Therefore, heat treatment of the formed carbonaceous fibers is not an acceptable technique for limiting the emission of electrons.
Moreover, since these processes involve direct formation of carbon on a substrate such as a cathode, mass production requires unique know-how, as well as special facilities and manufacturing techniques. Thus, due to such requirements, manufacturers of cathode materials generally do not employ such processes.
In recent years, carbon nanotubes having a diameter of some 10 nm or less have been studied as an electron-emitting material. A carbon fiber nanotube is a tube formed of graphite and typically has a diameter of 1-50 nm. Such a product can be formed by deposition on an electrode by arc discharge of a carbon electrode or by application of a high-intensity laser beam to a carbon electrode in a suitable atmosphere. The nanotube typically has one sharp end. See, for example, Chemistry Today, p.57, July, 1998.
Carbon nanotubes have chemical stability and high mechanical toughness, and applications thereof as electron sources for field emission are currently being investigated. For example, Saito et al. disclose in Ceramics, 33, (1998), No. 6, a fluorescent display device in which a number of carbon nanotubes are attached to a cathode plate. The authors indicate possible use of carbon nanotubes in a display device such as a low-power planar display device or an ultrafine color CRT.
However, no suitable industrial process for producing carbon nanotubes has yet been established, and thus inexpensive carbon nanotubes of stable quality have not been available in commercial quantities.
Recently, vapor-grown carbon fibers having a structure similar to that of carbon nanotubes and with a diameter on the order of several microns have been produced on a large scale. As disclosed in Japanese Patent Publication (Kokoku) No.04-24320 and Japanese Patent No. 2778434, the above type of carbon fiber is produced by spraying an organic compound in a reactor to thereby pyrolyze the compound. Precise examination of the thus-obtained fiber has revealed that the fiber is composed of planes of carbon atoms having a condensed ring structure concentrically grown around the longitudinal axis of the fiber. The fiber has a spherical closed end or a cut end having a cross-section in a plane approximately normal to the fiber axis.
If the fiber has a sharp end, carbon atoms of a condensed ring structure appear in the edge plane, to thereby enhance the field emission characteristic of the carbonaceous fiber when used as a field emission source.
However, a carbonaceous fiber having two sharp ends has not been found. Use of such a carbonaceous fiber as an electron-emitting material is expected to have an effect of increasing emission efficiency.
The present inventors have considered that a vapor-grown carbon fiber produced using an industrially established process can be employed in order to produce a carbonaceous fiber having sharp ends and which is suitable for use as an electron-emitting material.
In view of the foregoing, an object of the present invention is to provide carbonaceous fibers produced through a conventional process but which additionally have suitable sharp ends. Another object of the invention is to provide a process for producing such fibers on a large scale.
The present inventors have investigated a variety of methods for forming a sharp end on an existing type of carbonaceous fiber including subjecting the fiber to mechanical impact and effecting wear to the tip of the fiber, and have found that heating the carbonaceous fiber during formation in the presence of oxygen effectively forms sharp ends on the fiber. The present invention has been accomplished on the basis of this finding, to thereby provide a carbonaceous fiber having sharp ends.
Accordingly, the present invention provides a carbonaceous fiber having a structure such that planes formed of carbon atoms in a condensed ring structure are concentrically grown around the fiber axis, with the fiber having both ends sharp. Such a carbonaceous fiber may be characterized by d1/d0 less than 0.5 and L/d0 greater than 0.5, where d0 is the main diameter of the carbonaceous fiber, d1 is the diameter at the end of the carbonaceous fiber, and L is the distance from the end of the carbonaceous fiber to the point where the diameter starts to decrease. The carbonaceous fiber may have a hollow structure around and along the fiber axis.
The invention further provides carbonaceous fibers having a structure such that planes formed of carbon atoms in a condensed ring structure are concentrically grown around the fiber axis, and which comprise a mixture of carbonaceous fibers having sharp ends and carbonaceous fibers having one or two non-sharp ends. The carbonaceous fibers may have a hollow structure around and along the fiber axis. Such carbonaceous fibers may comprise over 10% of carbonaceous fibers having sharp ends. These carbonaceous fibers may have a hollow structure around and along the fiber axis.
The fibers of the invention can be produced by a process for producing carbonaceous fibers having sharp ends including vapor growing carbonaceous fibers composed of planes of carbon atoms in a condensed ring structure arranged concentrically around the longitudinal axis of the fiber, and then heating the resulting carbonaceous fibers to a temperature of 400-1200xc2x0 C. in the presence of oxygen. For this process, fired or graphitized carbon fiber can be used as the raw material.
According to another aspect of the present invention, carbonaceous fibers having sharp ends are produced by providing carbon nanotubes, and then heating the carbon nanotubes to a temperature of 400-1200xc2x0 C. in the presence of oxygen.